One of the most devastating disorders of old age is osteoporosis, which increases fracture risk and reduces quality of life for tens of millions of Americans. Available evidence suggests that both the quality and the quantity of fat consumed can have important effects on bone integrity in the young. Little research, however, has examined these effects with age. The proposed research will use aged rats to determine the effects of moderate- and high-fat diets rich in specific fatty acids on calcium excretion, hormonal activity, and bone composition. The overall hypothesis guiding this research is that consumption of diets rich in fat will have adverse effects on bone integrity in older individuals. These effects will be due to alterations in calcium excretion and growth hormone (GH) responsiveness, which will be affected differentially by the type of fat consumed. Three experiments will address three specific aims designed to determine the effects of moderate- and high-fat diets containing saturated and trans-fatty acids or n-6 fatty acids on 1) calcium excretion, 2) GH responsiveness, and 3) bone density, volume, and mineral content. Old rats will be compared among six diet conditions: 10 percent, 40 percent, and 78 percent shortening (rich in saturated and trans fatty acids), and 10 percent, 40 percent, and 78 percent borage oil (rich in n-6 fatty acids). Caloric intakes will be yoked to assure that vitamin, mineral and protein intakes do not vary between groups. It is predicted that calcium excretion will increase, and GH responsiveness, bone density, bone volume, and bone mineral content will decrease, as the fat content of the diet increases. It is further predicted that these effects will be greatest when saturated and trans fats are consumed. Given the dietary fat consumption patterns of many Americans, the increasing size of the aged population, and the high prevalence of osteoporosis in the United States, there is an urgent need to examine the impact that dietary fats have on skeletal integrity with age.